The basis for understanding the principle of body emissivity is the concept of the “Absolutely Black Body” that was defined by Kirchhoff in 1860. This is an ideal surface that neither reflects nor transmits light, but does absorb all incidental radiation, regardless of its direction and wavelength. However, an absolutely black body also emits this radiation from its surface ideally. The most accurate comparison is that of a cavity with an inner surface at a constant temperature, which communicates with the outside environment by means of an infinitely small hole, through which enters light, that is completely absorbed regardless of the internal surface of the cavity. In order to define the radiating capacity of the surface of the real body in comparison to the absolutely black body radiation, Kirchhoff defined the emissivity, ε, of the surface of the real body as the ratio between the thermal radiation at a given temperature, T, and the radiation of the absolutely black body at the same T under the same spectral and directional conditions. Absolutely black body emissivity=1.
In 1879 Stefan defined the relation that states that the radiation emitted by a body surface is proportional to the fourth power of the absolute T of the surface. This theory was later developed by Boltzmann, and in this manner a fundamental radiation law was defined, that states that E=C×T4, i.e. that the energy of radiation E of an absolutely black body at absolute T (° K) is equal to the product of the absolute T to the fourth power and C that represents the Stefan-Boltzmann constant. Based on this relation it is possible to divide all real bodies into two basic groups. The first group are “grey” bodies with an emissivity unrelated to the wavelength and equal to 0.8 and higher. This group includes all thermally non-conductive substances such as most colourless minerals, metallic oxides, ceramics, building materials and additionally most organic compounds and carbon. The second group includes all the coloured and highly thermally conductive materials, especially metals, the emissivity of which is lower in the same proportion as their surface is smoother. The emissivity of these compounds varies significantly in accordance with their wavelength (FIG. 1).
In 1896 Wien derived a formula for calculating the spectral characteristics of blackbody radiation as a function of T and the wavelength, which was adjusted by Planck in the year 1900 to constitute Planck's distribution law, which states that the intensity of radiation continuously changes in accordance with changes in the wavelength. By increasing T the total amount of energy radiated is increased and the peaks of the curves move into the range of shorter wavelengths, i.e. into the region of visible radiation (FIG. 2). The dashed line in FIG. 2 demarcates the values of maximum radiation for each T at a given wavelength. This brings us to the third basic law—Wien's displacement law, which can be expressed mathematically by the following equation: λ=2898/T, in which λ represents the wavelength. In this manner it is possible to calculate the wavelength in microns, which defines the maximum amount of energy radiated at a given T.
The three fundamental laws described above: the Stefan-Boltzmann law, Planck's distribution law and Wien's displacement law have been of significant help in many fields, including pyrotechnics for over a hundred years. In order to achieve the highest visibility of a flame in various lighting, signalling and tracer mixtures it is necessary to maximise the T of the flame, which is directly proportional to the heat of the reaction of the mixture minus the latent melting and boiling heat of the products and indirect proportional to the specific heat of the products. Therefore, we chose high-calorific fuel mixed with oxidising agents that have a high content of “active” oxygen that produce combustible products, the particles of which have the highest possible melting point and emissivity, approaching the maximum emissivity of an absolutely black body. Therefore, high-calorific metals have a unique function in these mixtures; Mg, for example, while it is in a gaseous state, is burned both on the account of the oxygen from the oxidising agent and atmospheric oxygen too, which maximally increases the volume and the surface area of the flame and produces a high fusible MgO, which has a high emissivity in the range of 500 nm—i.e. optimal for viewing with the human eye. These mixtures work within a temperature range of 1200-3500° C.
In roughly the early 1970's, in connection with continuing improvement to optical instruments for use not only in the area of visible radiation (400-760 nm) but also primarily in the area of long waves, invisible to the human eye, in the near-infrared radiation range (780-1400 nm), the special task became necessary of equipping units conducting night combat operations with devices for night vision, i.e. the development of a special tracer mixture together with the maximum suppression of the emission of visible radiation, operating in the near-infrared radiation range.
FIG. 2 shows which direction it is necessary to take in order to successfully solve this problem. It is based on a maximum reduction of the burning T while maintaining a sufficient quantity of suitable emitters in the combustion products, the emissivity of which most closely approximates that of an absolutely black body. In FIG. 2 it can be seen that this is an extremely difficult task; one that is just within the bounds of possibility for the pyrotechnics art. The reduction of the burning T and thereby also the heat of the reaction of the pyrotechnic mixtures has its limits, because the heat of the reaction in relation to the ignition T is one of the basic criterions for evaluation of a reliability and stability of burning, which is especially significant during burning in small cross sections—in cavities in devices made of highly conductive materials such as bullet jackets and, of course, at a very low ambient T. Too low a value of the “propagation index”—P.I.—i.e. the ratio of the heat of the reaction in cal/g and the combustion temperature in ° C. can then be the cause of the unsatisfactory operation or of the complete malfunctioning of the mixture in the bullet. A combustion T of around 500° C. is considered as a limit in pyrotechnics and mastering reliable combustion for such low temperatures, and especially under the above conditions, is possible only by reducing the ignition T by using highly reactive oxidising agents or fuel, which, however, undesirably increases sensitivity to mechanical impacts and thereby also handling hazards. Even when making every effort in order that these negative effects should be suppressed, the solution will always be borderline, as is demonstrated by the curve corresponding to T 800° K. (approximately 500° C.) in FIG. 2—burning will still to intervene in the visible spectrum region and the intensity of radiation in the infrared region will be limited by the sensitivity of night vision devices, the operating range of which lies between 400 and 1000-1500 nm.
In principle there is also the possibility of a different solution, based on the total absence of solid emitters in the flame. If the flame contains only gaseous components or vapours, the emissivity of which is negligible, this could have a significantly higher T without being visible to the naked eye. Such a flame occurs, for example, during the combustion of ethanol, which, with atmospheric oxygen, burns at T 1600° C. with the production of only gaseous products. For comparison, a candle flame at T 800-1000° C. containing nanoparticles of unburned carbon radiates at a much greater intensity and it was for this reason that it was determined as the basic unit of light intensity in the SI-1 cd/candela. In terms of pyrotechnics, however, the production of non-luminous flames is difficult to achieve.
Manufacturers of ammunition have already been trying to resolve this problem for 40 years. There are basically two types of mixtures emitting in the near-infrared region: mixtures designed for screening equipment against the IR guiding warheads of the enemy's weaponry and the tracers loaded into bullets of various calibres. In the case of the first category it is not so important whether these mixtures will also emit radiation in the visible region, and their composition is different—they also can contain powdered metals. The mixtures for tracers are obviously designed in accordance with the above principles and patterns and are based solely on the usage of non-metallic fuel.
The selection of oxidising agents, which are in principle inorganic oxides, peroxides or salts of inorganic oxyacids, is governed solely by their content of “active oxygen” and their reactivity—i.e. their capacity to split of oxygen in the presence of reducing agents at the lowest T as possible. The best oxidising agents for these purposes appear to be those with both a low content of active oxygen and high reactivity. These are already-known compounds and their selection is very limited. Of the group of metal oxides and peroxides only the metal peroxides and of the peroxides solely those of Ba, Sr and Zn satisfy these requirements. Peroxides, unlike oxides, contain the —O—O— bond, which is less strong than the bond of the Me═O type, in which Me is the metal. Peroxides of Ba and Sr, combinations of which most commonly occur in a number of patents from the 1970's to the 1990's are chemically unstable compounds, however, reacting with water already at a normal T in accordance with the reaction:BaO2+2H2O→Ba(OH)2+H2O2 
In addition, both these peroxides occur in the form of stable octahydrates, so if they are used in their anhydrous form they naturally have a tendency to reabsorb water again. Mixtures based on a combination of these peroxides with organic fuel and binders like the calcium resinate and phenolic resins types are described in U.S. Pat. Nos. 3,667,842, 5,639,984 and 5,811,724.
Unlike the peroxides listed above the peroxide of Zn is chemically completely inert, moisture-proof and, in comparison with the peroxide of Ba, environmentally friendlier. In addition, the oxygen is bound there by weaker covalent bonds; therefore it is more reactive in comparison with the Ba and Sr peroxides, which are ionic compounds. Compositions based on zinc peroxide in mixtures with potassium nitrate and a fuel based on alkali salts of organic acids, such as sodium salicylate together with binders based on cellulose derivatives or fluoroelastomers are described in US Patent No. 2006/0219339 A 1. Unfortunately these mixtures are applicable only to medium-calibre ammunition—i.e. 12.7 and higher. The most likely explanation for this limitation is the incapability of these mixtures to burn steadily in significantly smaller cross sections.
The choice of inorganic salts is also considerably restricted—chlorates and perchlorates are completely unsuitable for these purposes, since a mixture with an organic fuel under high pressure explodes in the gun barrel. Ammonium perchlorate releases extremely corrosive hydrogen chloride and, furthermore, it is considered unstable in accordance with the modern evaluation criteria. Of the nitrates only usable are those that are moisture-proof with a sufficiently low point of decomposition. It is therefore necessary to exclude, as extremely hygroscopic, ammonium nitrate, U.S. Pat. No. 5,587,552, and barium and strontium nitrates with overly elevated dissociation points—above 600° C. That leaves only the nitrates of rubidium and caesium, U.S. Pat. No. 3,773,223, as emitters in the near-infrared region, but these are considered as being potentially carcinogenic, and the above-mentioned potassium nitrate that actually does meet the requirements defined. It is environmentally friendly, moderately reactive, and relatively less hygroscopic. Even our company has developed infrared tracers based on mixtures of this oxidising agent with salts and derivatives of aromatic mono- and dicarboxylic acids. These mixtures, which are similar to the black powder, are based on the principle of a cold fuel that consists of organic compounds with a high content of molecular oxygen, cooled by a possible decarboxylation, which is an endothermic reaction. In the same manner as black powder does, they burn reliably even in small cross sections, and their reactivity is directly proportional to the content of oxygen in the molecule. The emitter in the near-infrared region consists predominantly of solid or liquid potassium carbonate. The burning is invisible to the naked eye but sufficiently visible when utilising night vision devices. The problem with this is the unpredictable behaviour of some of these mixtures, especially those containing fuel with a high content of oxygen under high pressures generated in gun barrels. Explosive burning probably takes place, in addition to tracer burn-out, already in the barrel or near the muzzle, which is mistakenly interpreted as a misfire. Though the use of fuel with a lower content of bound oxygen and therefore less reactivity eliminates this problem, but these fuels require the use of pyrotechnic igniters. However, the development of a non-peroxide type of igniter that would operate at sufficiently low temperatures to be invisible to the naked eye failed. Our company tried to solve this problem by using gasless time-delay compositions, the linear burning rate of which independent on the pressure in the weapon and that would be able to delay the ignition of the main composition to a distance sufficiently remote from the muzzle. Dozens of thermitic mixtures, based on the reaction between metal oxides and inorganic non-metallic or metallic fuels were tested. The preparation of a composition that would work reliably at a sufficiently low T and that would thereby be invisible to the naked eye was unsuccessful, however. The excessive ignition T of these mixtures, in principle caused by the lower reactivity of the oxides (500-600° C.), simply makes it impossible to use sufficiently “cold” compositions—see the propagation index—P.I. Other salts of inorganic oxyacids, such as chromates and permanganates, are also unusable for this purpose; the chromates have too high a decomposition T-800° C. or above and, in addition, all the salts of hexavalent chromium are classified as toxic and opposite to this highly reactive permanganates are very unstable.
From the above discussion it is clear that the options available for the use of the known oxidising agents have been practically exhausted and it is unlikely that in the foreseeable future there will be a breakthrough in the field of inorganic oxidising agents and for this reason it is necessary to focus on the search for and development of a special fuel.